The successful use of laminar flow to reduce drag on airplanes has been inhibited in the past by the inability of early airframe manufacturing techniques to meet the roughness and waviness requirements for stable laminar flow to occur. Modern airframe construction such as bonded or milled aluminum skins or composite skins are sufficiently free from significant roughness and waviness for successful use of natural laminar flow for drag reduction. The currently accepted natural laminar flow wing design practices, however, exclude the installation of wing-leading edge devices in order to preserve the smoothness of the leading edge so that the laminar flow can be maintained.
Even in the absence of leading edge devices such as slats, flaps, or de-icers, the production of the airfoil wherein leading edge structure interfaces with the upper and lower skin surfaces of the airfoil often results in an orthogonal step (forward or aft facing) or a gap. These steps or gaps on current airplanes manufactured to modern production tolerances are sufficiently large to cause the laminary boundary layer to transition to a turbulent boundary layer at or near the step or gap, thus greatly increasing wing drag.
Various techniques have been tried to overcome the above mentioned difficulties; however, only one previous known method has been utilized to successfully overcome the problem. This method is the use of suction upstream and/or downstream of a step or gap. Using this method, even if the step or gap which causes the transition is of considerable magnitude, it is possible to apply sufficient suction to relaminarize a turbulent boundary layer downstream of the step or gap. There are many problems associated with the suction technique for maintaining laminar flow which include its power requirements, cost, maintainability, and complexity. The active boundary layer suction technique utilizes suction of air through small slots or tiny holes in porous surfaces. These slots must be very accurately machined or the holes carefully drilled (by electron or laser beam techniques, for example). The suction system must be powered by a suction pump and connected to internal ducting for suction of the air mass flow. All of these system design, manufacturing, and component requirements combine to make the suction method expensive, relatively inefficient, and complex. The suction technique also results in an aerodynamic disadvantage as the laminar boundary layer is thinned by the suction. This thinner laminar boundary layer is then more sensitive to transition due to roughness; for example, caused by ice or insect debris which often occur on the leading edge region of the airfoil. Further, the suction slots or holes must be kept free from dust, dirt and debris which entails considerable maintenance cost.